What can they do to drive sales for a sequal? Zoo Tycoon had a massive up grade in the game. Is flying bird enough? not for me.
I agree, I really don't see that there's enough they could add in to upgrade it for a sequel for quite a while. Offhand, I can only think of four major things that are missing and/or have been included in past zoo builder games.
1. Flying birds. Really the jury is out on this one still. Hypothetically they should have the ability to do it, but will they?
2. True aquatics. Looking potentially promising given some of the files recently discovered. Easily one of the biggest mod categories, we'll have to wait and see if Frontier gives us anything official.
3. Extinct animals. While often included in past zoo builders, I doubt they would ever come officially to PZ. Given Frontier is releasing JWE2, it makes zero sense to include extincts in an established game and steal the thunder off the new game.
4. First person keeper mode. Definitely a major perk of ZT2, and I was a little surprised when PZ didn't follow suit. Whether this is something addable in a future major update I don't know, and frankly I don't really think we'll get it at this point. Which is a shame, since even though we can angle the camera to mimic the idea, we can't actually care for the animals ourselves.
Given that points 1 and 2 may still arrive, and 3 is almost a certain no, there's really not a ton of justification for a sequel in my opinion. That said, JWE recieved a sequel only 3ish years later so who knows?